Framebuffer, mmap(), hanging in D state, root FS unmount failure.

[Mark J Roberts <mjr@znex.org>]

#include <assert.h>
#include <sys/mman.h>
#include <fcntl.h>
int main(void)
{
	char *p;
	assert((p = mmap(0, 1, PROT_READ|PROT_WRITE, MAP_SHARED, open("/dev/fb/0", O_RDWR), 0)) != MAP_FAILED);
	p[4096] = 0; /* this hangs */
	return 0;
}

When I run this on my 2.4.17rc2aa2 kernel with a Voodoo3000
framebuffer, the process hangs forever in D state. ps and top will
then hang the same way when they read the /proc/pid files for the
hung process. And my root filesystem won't unmount.

It only happens when PROT_READ|PROT_WRITE is specified - when I use
only PROT_WRITE, the program segfaults like you'd expect.... but
once the PROT_READ|PROT_WRITE version has hung, PROT_WRITE-only
versions will also hang.